Enclosures submerged at great depth must withstand considerable hydrostatic pressures, which causes mechanical structural stresses of said enclosures.
In certain cases, the interior of the enclosure may be in communication with the exterior, with the result that there is always a pressure equilibrium and the walls of the enclosure do not have to withstand any pressure. However, in numerous cases, it is not possible to allow the water to penetrate in the submerged enclosure, inside which are located electrical or electronic apparatus or circuits which cannot be placed in contact with water or with an intermediate liquid.
In such cases, the interior of the enclosure is placed in communication with a deformable bag which is filled with gas and which is submerged at the same time as the enclosure, with the result that the gas located in the enclosure is constantly maintained at hydrostatic pressure.
In the following specification, reference will be made to embodiments concerning piezo-electric transducers which compose for example a sonar antenna which must be submerged to very great depths, of the order of several hundreds or thousands of meters. It is specified that this choice is not limiting and that the present invention may be applied to other submerged enclosures.
At the present time, the submerged transducers comprise a stack of piezo-electric elements, placed inside a water-tight housing which is connected by a pipe to a bag or bladder of deformable elastomer which is placed in an enclosure for mechanical protection which communicates with the ambient medium. If the bladder is filled with air at atmospheric pressure and if the volume of the housing of the transducer is relatively large, such a system can compensate the hydrostatic pressure only for a limited depth of immersion. In fact, let Vo be the initial volume of the bladder, V.sub.1 the volume of air in the housing, Po the initial pressure and P the hydrostatic pressure when all the air contained in the bladder has passed in the housing, the following may be written: EQU Po(Vo+V.sub.1)=P.sub.1 V.sub.1,
hence: ##EQU1##
In order to attain a depth of immersion of 500 m which corresponds to a hydrostatic pressure of 50 bars, a bladder must therefore be used whose volume is equal to 49 times that of the assembly of the transducer housings if the initial pressure Po=1 bar. Such a bladder volume is generally not compatible with the dimensions admitted.
In order to reduce the volume of air, compensation systems have been used, composed of deformable bladders or bags which are pre-inflated under a pressure Po equal to several times atmospheric pressure. If, for example, Po=5 bars and if Vo/V.sub.1 =19, the hydrostatic pressure limits P.sub.1 =Po.times.20, or P.sub.1 =100 bars, i.e. a maximum depth of immersion of 1000 meters may be attained.
This solution means that, from the beginning of immersion, the housing of the transducer is subjected to an internal pressure and must be able to withstand it.
The housings having a considerable volume cannot withstand a high internal pressure, this limiting the depths of immersion which may be attained by a system of pressure compensation comprising preinflated bladders.
Another solution consists in connecting the housing of the transducer to a compressed air canister provided with a pressure reducing valve, like those used for diving.
In that case, during descent of the transducer and of the canister, the pressure reducing valve delivers air at a pressure which corresponds substantially to hydrostatic pressure. Even if the volume of the canister is small with respect to the volume of the housing, a considerable depth of immersion may be attained if the canister contains highly compressed gas. For example, if the initial pressure in the canister is 50 bars and if the ratio Vo/V.sub.1 =1/10, a hydrostatic pressure of 55 bars may be attained, viz. a depth of 550 meters. On rising to the surface, an excess pressure is created inside the housing and a valve or the pressure reducing valve allows the excess air to escape.
This solution therefore brings about the formation of air bubbles when rising to the surface, which is contra-indicated in the case of submerged sonar antennas used for military purposes.
Moreover, in the case of poor functioning of the pressure reducing valve, there is a risk of creating in the housing an excess pressure which it cannot withstand, hence a risk of destruction of the housing and of the equipment that it contains.
Moreover, in the case of the transducers forming part of a submerged machine which is towed by a ship, the depth of immersion continually varies, this causing an escape of air bubbles each time the transducer rises, which leads to a more rapid air consumption than the consumption expected and possibly bringing about a difference in pressure between the exterior and interior of the housing greater than the mechanical strength thereof.
It is an object of the present invention to provide means for establishing the equilibrium of the gas pressure inside a submerged enclosure with the hydrostatic pressure by means of gas-filled deformable bladders or bags which are submerged at the same time as the enclosure and which enable considerable depths of immersion to be attained without having to use large-volume bags nor to pre-inflate the enclosures on the surface.
A process according to the invention is of the type in which an enclosure and a gas-filled deformable bag which communicates with said enclosure are submerged simultaneously.